A lot of people wonder why they need impact windows to protect their homes against hurricanes. What happens when it’s impact windows vs hurricanes as opposed to regular windows versus a hurricane?
How does the way that an impact window is built and installed affect its ability to withstand the stresses of a major hurricane? Why should you use impact windows in your home regardless of hurricane risks?
Let’s go over a tiny bit of the science of how hurricanes affect windows and the innovations that make hurricane impact windows so resilient against the extreme forces of Mother Nature:
Impact Windows vs Hurricanes: Wind Loads
The first thing that any impact window design has to take into account is the ‘wind load’ created by high-speed winds. If you’ve ever stood outside on a breezy day, you’ve personally felt the impact of wind load on your body.
While invisible to the naked eye, air has countless particles of oxygen, nitrogen, and other chemicals that, when they move, create the breeze you’re experiencing. The faster these particulates in the air move, the more pressure your body will experience. The same is true for windows.
While air densities vary depending on elevation (the closer to sea level, the denser the air tends to be), the default assumption used by omni calculator is an air density of 0.0765 pounds per cubic foot (lb/ft3). The average human body has a total surface area of about 17 to 20 square feet (though that can vary based on height and weight).
Assuming you’re average height & weight for a man and are taking the wind head-on, that means you have 10ft2 of your body being pressed by the wind. So, at a wind speed of 30 mph, you’d be subjected to roughly 23 pounds of force on your body. At a wind speed of 130 mph, you’d feel 432.2 pounds of force pressing on your body.
That’s twice the weight of a healthy adult pressing on you. Well, the same goes for your windows.
During hurricanes, winds can reach incredibly high speeds. A Category 5 storm has winds consistently in excess of 156 mph. At that speed, an object with a surface area of 10ft2 will experience 622.4 pounds of force pressing on it… that’s about three times the weight of a healthy adult male… or more than the weight of a grizzly bear (600 lbs, according to the University of Montana).
The challenge with wind loads is that they represent sustained pressure over a long period of time—they don’t account for sudden gusts of wind or for the impact force of flying debris. Even then, the sheer force of wind pressing against windows can easily cause them to fail if they aren’t built for that kind of stress.
Non-impact glass can generally only withstand about 15-30 lbs/ft2 worth of pressure before failing… and hurricane-force winds put dozens of pounds/ft2 of pressure on your windows. As the wind load increases, the glass in a non-impact window is pressed inward, towards the home. After a few moments of pressure beyond its tolerance, the glass flexes too much, and a crack forms.
Once broken, the integrity of the window collapses completely as high-speed winds send pieces of the non-impact window flying into the home… along with storm debris, rainwater, and storm surge.
With the new opening, the wind blows into your home, but doesn’t immediately have anywhere to go. Pressure builds up against your interior walls and the underside of your roof until the pressure differential between the windward side of your home and the leeward side grows too severe, and a critical part of your home’s structure catastrophically fails. In most cases, this will be the roof, which peels off and leaves large parts of the home’s interior exposed to wind, rain, and worse.
As noted by the Whole Building Design Guide, “the breaching of a small window is typically sufficient to cause full pressurization of the building’s interior.”
Impact windows prevent this chain of events by being built to withstand extreme wind loads without cracking. Even if the top layer of glass is compromised, an impact window has a semi-flexible laminate interlayer and more layers of glass beyond it to maintain the seal of your home against the wind pressure exerted by a storm.
Speaking of leeward and windward parts of your home, let’s explain the effects of negative pressure on your home:
Impact Windows vs Hurricanes: Negative Pressure
Hurricanes really do suck. No, literally. When the wind hits one side of your home and passes over it, it creates a zone of negative pressure on the other side that pulls on your home’s structure—including your windows.
The faster the wind, the stronger the suction force on the ‘leeward’ side of your home (the side opposite where the wind is coming from).
This pressure from suction acts on your windows just like the wind load from the windward side does. However, instead of sending pieces of broken window immediately into your home, your home’s windows blow out and the home becomes depressurized instead of overpressurized.
This pulls down on your ceiling and roof as air rushes out of the home, and pulls on the windward wall while it’s still being pushed from the other side by high-speed winds.
In extreme cases, the building frame may collapse inwards or the windward wall may fall over, taking the structure of the home with it.
This is one of the reasons why you should never open any windows or doors in the middle of a major storm, regardless of which wall the opening is on!
Impact windows help prevent this issue by being designed and tested to withstand wind pressure from multiple directions. This is achieved via cyclic pressure tests, where the window is subjected to extreme wind pressure and suction in alternating directions rapidly. This test is a core part of the Missile Impact D testing standard,
After a missile impact, the window being tested is subjected to cyclic pressure loading of inward and outward pressure loads at varying intensities for hundreds or even thousands of cycles.
This testing helps regulators determine what the actual positive and negative pressure a window can take without leaking is.
If you’ve ever read an impact window specification sheet, odds are that you’ve seen a rating on there called design pressure rating or DP rating with a ± or +/- symbol with some numbers next to it. This statistic signifies the inward and outward design pressure tolerances of the window.
For example, a window with a DP rating of (TAS +60/-70) is verified as being rated to take up to 60 pounds per square foot of inward pressure and 70 psf2 of outward pressure.
When it comes to withstanding negative pressure, window frames are extra important. They not only need to be solid enough to avoid warping, cracking, or breaking from the incredible pull of a hurricane, they also need to be securely fastened in the window opening so the whole assembly doesn’t get pulled out of the wall by the suction force of a hurricane.
Impact Windows vs Hurricanes: Flying Debris Impacts
As the comedian Ron White once said: “It’s not that the wind is blowing, it’s what the wind is blowing!” For an impact window, withstanding incredible, but consistent, wind pressure isn’t a challenge.
However, during a storm, the wind can pick up and throw objects at your home. The faster the wind, the larger, heavier, and faster these objects will be.
How much wind is needed to move something like a car? There’s a mathematical formula that can give you a rough estimate: it’s the square root of 195 times (the object’s weight in pounds divided by its surface area in square feet). ABC7 Chicago used the formula to calculate it based on a car weighing 2871 lbs and having a surface area of 69ft2 in total. At that weight and surface area, the car can be moved by wind speeds as low as 90 mph.
2871 ➗ 69 = 41.6086956522. 41.6086956522 x 195 = 8113.696 (rounded to the nearest thousandth). The square root of that is 90.076 (rounded to the nearest thousandth).
Basically, the larger the object is and the lighter it is, the easier it is for high-speed winds to move it. When the wind speed needed to pick up an object is far in excess of that object’s weight, then the object won’t just move, it will get picked up and thrown, violently.
To resist impacts from flying debris without breaking and allowing the envelope of the home to become compromised, impact windows use multiple layers of tempered glass with laminate interlayers and a reinforced frame.
This heavy-duty construction serves the window well outside of hurricane conditions, too! Impact-rated windows can resist accidental baseball impacts and other events without breaking. They can also deter would-be break-in artists since they don’t simply shatter open at the drop of a rock.
Impact Windows vs Hurricanes: Why Installation Quality Matters
When it comes to getting impact windows for your home, it isn’t enough to just get strong windows. You need to get a high-quality installation that maximizes the durability of your windows.
Poor window installation work where installers cut corners means playing a game of Russian Roulette with your home when hurricanes come into town. It doesn’t matter how strong the window glass and frame are if the installer didn’t secure the frame to your window opening properly and the suction force of a major hurricane rips the whole assembly out of your wall!
That’s why you need to get expert window installation service you can trust from a reliable installer who doesn’t cut corners! Karoly Windows & Doors has over a quarter century of experience installing impact windows for homes throughout the Greater Tampa Bay area, consistently earning 5-Star ratings across Google, Angi, Yelp!, and other review platforms for our consistent dedication to doing things the right way, every time.
Not only do we have our own in-house team of trained and certified professional impact window installation specialists (unlike some companies that outsource to unqualified subcontractors to save a buck), we use top-quality shims, caulking, insulation, and tools to ensure that every installation is a close to perfect as possible.
We’re so confident that your window installation will be flawless that every install comes backed by a 25-year labor warranty and a 50-year material warranty. With Karoly, you can rest easy knowing that your impact windows will be there to protect you and yours for decades to come without fail.
Reach out today to request a free in-home impact window consultation and quote for service!
